Periodic breathing refers to abnormal respiration patterns that alternate between hypopnea (i.e. diminished breathing) and hyperpnea (i.e. fast, deep breathing.) One form of periodic breathing is Cheyne-Stokes Respiration (CSR), which can occur in patients with congestive heart failure (CHF). CSR typically occurs while the patient is sleeping. Briefly, CSR arises principally due to a time lag between blood carbon dioxide (CO2) levels sensed by the central nervous system and the blood CO2 levels. With CHF, poor cardiac function results in poor blood flow to the brain such that the central nervous system responds to blood CO2 levels that are no longer properly representative of the overall blood CO2 levels in the body. Hence, the central nervous system triggers an increase in the depth and frequency of breathing in an attempt to compensate for perceived high blood CO2 levels; whereas the blood CO2 levels have already dropped. By the time the central nervous system detects the drop in blood CO2 levels and slows respiration in response, the blood CO2 levels have already increased. This cycle becomes increasingly unbalanced until respiration periodically alternates between hypopnea and hyperpnea. Typically, the time from one period of hyperpnea until the next is about one minute.
The periods of hypopnea occurring during CSR can be sufficiently pronounced that breathing completely ceases during hypopnea, i.e. the patient suffers from episodes of frank apnea. The episodes of apnea can cause the patient to awaken due to blood O2 depletion (i.e. hypoxia). Arousal from sleep usually lasts only a few seconds, but such brief arousals can occur hundreds of times a night, thus significantly disrupting continuous sleep leading to excessive sleepiness during the day, which diminishes quality of life. Worse, the frequent interruptions from sleep can prevent the patient from achieving rapid eye movement (REM) sleep, which is needed. In this regard, REM sleep does not usually occur until after some period of the sustained Stage 3/Stage 4 sleep. CSR usually arises during Stage 3 sleep. Hence, repeated sleep interruptions occurring during CSR typically prevent the patient from achieving any REM sleep or, at least, an insufficient amount of REM sleep is achieved over the course of the night. Whether a patient is actually awakened due to apnea occurring during CSR depends on various factors. In some cases, CSR is sustained without sleep interruption. Such cases are referred to herein as “sustained CSR”. In other cases, herein referred to as “non-sustained CSR”, sleep interruptions are frequent and debilitating.
In view of the adverse consequences of sleep interruption during CSR, it is highly desirable to provide techniques for treating CSR so as to prevent possible sleep interruption. One particularly promising solution is to employ an implantable medical system for the detection and treatment of CSR. This may be achieved by, for example, applying diaphragmatic pacing via phrenic nerve stimulators during periods of apnea to improve respiration during apnea so as to equalize the cyclic blood chemistry variations occurring during CSR. Alternatively, overdrive pacing therapy may be applied, also in an attempt to mitigate CSR and reduce cyclic blood chemistry variations to prevent repeated sleep interruptions. The implantable medical system may utilize a pacemaker or ICD for use as a controller to coordinate the detection of CSR and delivery of therapy in response thereto. Pacemakers and ICDs are usually implanted primarily for use in applying cardiac therapy for treating arrhythmias or for delivering cardiac resynchronization therapy in an effort to alleviate CHF. However, many patients who are candidates for pacemakers or ICDs also suffer from CSR and hence could benefit from additional functionality directed to the detection and treatment of CSR.
An example of a technique for performing diaphragmatic pacing during the hypopnea/apnea phase of CSR using an implantable medical system incorporating a pacemaker is set forth in U.S. Pat. No. 6,415,183 to Scheineret al., entitled “Method and Apparatus for Diaphragmatic Pacing.” Another technique for delivering therapy in response to CSR is set forth in U.S. Pat. No. 6,641,542 to Cho et al., entitled “Method and Apparatus to Detect and Treat Sleep Respiratory Events”. A technique for delivering overdrive pacing therapy in response to sleep apnea, including apnea caused by CSR, is set forth in U.S. Patent Application: 2003/0153954 A1 of Park et al., entitled “Sleep Apnea Therapy Device Using Dynamic Overdrive Pacing”, which is incorporated by reference herein.
Techniques for treating CSR in an effort to prevent sleep interruptions during CSR using an implantable medical system are promising. However, considerable room for improvement remains. In this regard, many CSR treatment techniques operate to deliver therapy continuously during CSR, without regard to whether sleep is likely to be interrupted. With some techniques, once an episode of CSR has been detected, overdrive pacing is applied continuously throughout the entire episode of CSR. Other techniques operate to deliver diaphragmatic pacing during all episodes of apnea occurring during CSR. Therapy is thereby provided even circumstances where sleep might not actually be interrupted due to CSR. In some cases, the amount of respiration achieved during each burst of respiration during CSR (i.e. during each hyperpnea phase) is a sufficient to sustain the patient through a period of hypopnea/apnea until the next burst of respiration. Hence, although the alternating CSR pattern of hyperpnea and hypopnea/apnea may be sustained, the patient does not awaken due to hypopnea/apnea, and hence the above-described problems arising because of frequent sleep interruption do not occur.
Thus, in at least some cases, CSR therapy is delivered when it is not required or, at least, therapy is delivered that is more aggressive than is otherwise necessary. Either can have adverse consequences. Providing continuous aggressive overdrive pacing throughout all episodes of CSR can unnecessarily diminish power supplies of the implantable system and can potentially adversely affect the hemodynamic balance of the patient—due to the continuously elevated heart rate. Moreover, continuous aggressive overdrive pacing of the atria can potentially trigger atrial fibrillation. Providing continuous diaphragmatic pacing throughout all episodes of apnea occurring during CSR can likewise diminish power supplies of the implantable system. Moreover, frequent diaphragmatic pacing via the phrenic nerves can potentially damage the nerves.
Accordingly, it would be desirable to provide improved treatment techniques for CSR or other forms of periodic breathing for use with implantable medical systems, which address these various concerns.